Tachyarrhythmias are abnormal heart rhythms characterized by a rapid heart rate, typically expressed in units of beats per minute (bpm). They can occur in either chamber of the heart (i.e., ventricles or atria) or both. Examples of tachyarrhythmias include ventricular tachycardia, ventricular fibrillation, atrial tachycardia, atrial flutter, and atrial fibrillation. Tachycardia is characterized by a rapid rate, either due to an ectopic excitatory focus or abnormal excitation by normal pacemaker tissue. Fibrillation occurs when the chamber depolarizes in a chaotic fashion with abnormal depolarization waveforms as reflected by an EKG.
An electrical shock applied to a heart chamber can be used to terminate most tachyarrhythmias. The electric shock terminates the tachyarrhythmia by depolarizing all of the myocardium simultaneously and rendering it refractory. A class of cardiac rhythm management devices known as an implantable cardioverter/defibrillator (ICD) provides this kind of therapy by delivering a shock pulse to the heart when the device detects fibrillation. ICDs can be designed to treat either atrial or ventricular tachyarrhythmias, or both, and may also incorporate cardiac pacing functionality for delivering either bradycardia pacing or anti-tachycardia pacing (ATP). In ATP, the heart is competitively paced with one or more pacing pulses in an effort to interrupt the reentrant circuit causing the tachycardia.
The most dangerous tachyarrhythmias are ventricular tachycardia and ventricular fibrillation, and ICDs have most commonly been applied in the treatment of those conditions. ICDs are also capable, however, of detecting atrial fibrillation and delivering a shock pulse to the atria in order to terminate the arrhythmia. Although not immediately life-threatening, it is important to treat atrial fibrillation for several reasons. First, atrial fibrillation is associated with a loss of atrio-ventricular synchrony which can be hemodynamically compromising and cause such symptoms as dyspnea, fatigue, vertigo, and angina. Atrial fibrillation can also predispose to strokes resulting from emboli forming in the left atrium. Although drug therapy and/or in-hospital cardioversion are acceptable treatment modalities for atrial fibrillation, ICDs configured to treat atrial fibrillation offer a number of advantages to certain patients, including convenience and greater efficacy. (As the term is used herein, atrial fibrillation should also be taken to include atrial flutter, which although clinically distinct, has similar consequences and may be treated similarly.)
Although atrial fibrillation can be successfully treated with electrical therapy from an implantable cardiac rhythm management device, it would be preferable to prevent an episode of atrial fibrillation from occurring. Another problem associated with defibrillation shock therapy is early recurrence of atrial fibrillation or ERAF. ERAF is defined as the recurrence of atrial fibrillation within a few minutes after successful cardioversion with atrial shock therapy. Certain patients are more prone than others to experience ERAF, and these patients may experience difficulty with repeated atrial defibrillation therapy. Reducing the incidence of ERAF would improve the efficacy of atrial defibrillation by electrical therapy and expand the population of patients for whom it is an acceptable therapy option.